| Abstract
| - The bipolar/monopolar conformational transitions of the hydroxyoctadecanoic acids (HOAs) [6-hydroxy-9-hydroxy- and 12-hydroxyoctadecanoic acid (6HOA, 9HOA, and 12HOA)] plus those of the methyl andethyl esters of 12HOA have been studied using classical surface pressure (Π)/area per molecule (A) isothermsand fluorescence microscopy techniques. Most studies were carried out using identical compressional rates(5 Å2 mol-1 min-1) and identical amounts of a fluid-phase soluble fluorescent probe 1-palmitoyl-2-[6-(7-nitro-2-1-3-benzoxadiazol-4-yl)amino] caproyl phosphatidylcholine (NBD-PC); however, the effect of bothcompressional rate and probe concentration was investigated. For 6HOA, condensed-phase domains areroughly diamond-shaped with both nucleation and apparent crystal growth being observed. Larger,essentially hexagonal, crystals were observed for 9HOA. For 12HOA the extent of nucleation was muchgreater, producing elongated needlelike crystals quite different from either 6- or 9HOA. This differenceis ascribed to a greater stability of the condensed state, possibly due to better packing and enhancedhydrogen bonding. The highly brittle nature of the needlelike crystals support this interpretation. On thebasis of the π/A isotherms the 12HOA methyl ester condensed state shows little indication of diminishedstability vis-à-vis 12HOA, but the ethyl ester does. However, both esters show substantially larger crystals,indicating that growth is favored over nucleation, and it is proposed that the reduced hydrogen bondingof the ester polar group is a primary cause.
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